A dissident Russian journalist reported murdered in Kiev has dramatically reappeared alive in the middle of a briefing about his own killing by the Ukrainian state security service.

Ukrainian authorities had said on Tuesday that Arkady Babchenko, a 41-year-old critic of President Vladimir Putin and of Russian policy in Ukraine and Syria, had been shot dead at his flat and that his wife had found him in a pool of blood.

His reported murder touched off a war of words between Ukraine and Russia, a flurry of condemnations from European capitals and Washington, and shivers through the journalistic communities in both countries.

But on Wednesday, an emotional Babchenko appeared before reporters saying he had been part of a special Ukrainian operation to thwart a Russian attempt on his life, and that he was fine.

"I would like to apologise for what you have all had to go through," Babchenko, who looked on the verge of tears at times, told reporters. "I'm sorry, but there was no other way of doing it. Separately, I want to apologise to my wife for the hell that she has been through."

Babchenko's reappearance elicited gasps, then cheers and applause from journalists at the briefing.

He went on to thank the Ukrainian Security Service, the SBU, for saving his life and said the most important thing was that what he called other big acts of terror had been thwarted.

He did not specify what those planned acts were, but the SBU said it had received information about a plot to kill 30 people in Ukraine, including Babchenko, but had managed to prevent it. The security service declined to say who the other 29 people were.

A Ukrainian citizen recruited by Russia to find someone to kill Babchenko, the SBU said. He was given $40,000 to organise the murder, $30,000 for the killer and $10,000 for being an intermediary.

"We managed not only to break this cynical provocation, but also to document the preparation of this shameful crime by Russian special services," SBU chief Vasyl Hrytsak said.

General Prosecutor Yuriy Lutsenko, who appeared alongside Babchenko, said it was necessary to fake the journalist's death so that the organisers of the plot to kill him would believe they had succeeded. The Russian Foreign ministry said it was happy Babchenko had turned out to be alive after all, but described the stunt as an "anti-Russian provocation".